Understanding Dipole Moments in NH₃ and NF₃

The question asks why the dipole moment of ammonia (NH₃) is greater than that of nitrogen trifluoride (NF₃), even though fluorine is more electronegative than hydrogen. To understand this, we need to look at the structure of these molecules and how individual bond dipoles and lone pair dipoles contribute to the overall molecular dipole moment.

Molecular Structure and Polarity

Both NH₃ and NF₃ have a trigonal pyramidal geometry around the central nitrogen atom. This shape is due to the presence of three bond pairs and one lone pair of electrons on the nitrogen atom, resulting in sp³ hybridization of nitrogen. The lone pair occupies one of the tetrahedral positions.

• In NH₃, the central nitrogen atom is bonded to three hydrogen atoms. Nitrogen is more electronegative than hydrogen. Thus, each N-H bond is polar with the bond dipole pointing from hydrogen towards nitrogen ($\text{H}^{\delta+} \rightarrow \text{N}^{\delta-}$).
• In NF₃, the central nitrogen atom is bonded to three fluorine atoms. Fluorine is more electronegative than nitrogen. Thus, each N-F bond is polar with the bond dipole pointing from nitrogen towards fluorine ($\text{N}^{\delta+} \rightarrow \text{F}^{\delta-}$).

Contribution of Bond Dipoles and Lone Pair Dipole

The overall dipole moment of a molecule is the vector sum of all the individual bond dipoles and any dipole moment contributed by lone pairs of electrons.

• Lone Pair Dipole: In both NH₃ and NF₃, there is a lone pair of electrons on the nitrogen atom. This lone pair contributes an orbital dipole moment that points away from the nitrogen atom, in the direction of the lone pair density.
• Vector Addition: The molecular dipole moment ($\mu$) is the vector sum of the individual bond dipoles ($\mu_{\text{bond}}$) and the lone pair dipole ($\mu_{\text{lone pair}}$). $\mu_{\text{molecule}} = \sum \vec{\mu}_{\text{bond}} + \vec{\mu}_{\text{lone pair}}$.

Analyzing NH₃

In NH₃, the three N-H bond dipoles point towards the nitrogen atom. The lone pair dipole also points away from the nitrogen atom, in the same general direction as the resultant of the three N-H bond dipoles (upwards, along the axis of symmetry). Since both the bond dipoles' resultant and the lone pair dipole point in the same direction, they add up, resulting in a large net dipole moment for NH₃ (approximately 1.46 D).

Analyzing NF₃

In NF₃, the three N-F bond dipoles point away from the nitrogen atom, towards the more electronegative fluorine atoms. The lone pair dipole on nitrogen points away from the nitrogen atom (upwards, along the axis of symmetry). The resultant of the three N-F bond dipoles points downwards (towards the base of the pyramid formed by the fluorine atoms), which is in the opposite direction to the lone pair dipole moment. Therefore, the lone pair dipole and the resultant of the N-F bond dipoles partially cancel each other out. This cancellation leads to a much smaller net dipole moment for NF₃ (approximately 0.23 D).

Reason for the Difference

Even though F is more electronegative than H, leading to more polar N-F bonds compared to N-H bonds, the spatial arrangement and the directional contribution of the lone pair are crucial. In NH₃, the lone pair dipole reinforces the bond dipoles. In NF₃, the lone pair dipole opposes the resultant of the bond dipoles.

Therefore, the reason the dipole moment of NH₃ is greater than that of NF₃ is that the orbital dipole in NF₃ (due to the lone pair) is opposite in direction to the resultant dipole moment of the N-F bonds, causing a cancellation effect.

Let's look at the options again:

• Presence of extra lone pair in NF₃ - Both have one lone pair on N. Incorrect.
• Repulsion and steric hindrance between N and F in NF₃ - Not the primary reason for the dipole difference in direction/magnitude comparison with NH₃. Incorrect.
• The orbital dipole in NF₃ is opposite in direction to the resultant dipole moment of N-F bonds - This accurately describes the cancellation effect explained above. Correct.
• Total charge on NH₃ is positive - NH₃ is a neutral molecule with zero net charge. Incorrect.